44 CARL GOTTFRIED HARTMAN 



thickness of the albumen layer made it impossible to distinguish 

 the thick from the thin-walled portion of the blastocyst. Hence, 

 some eggs were sectioned equatorially, others meridionally, and 

 still others obliquely. Measurements of cells through the series, 

 however, indicated that in some of the blastocysts with about 

 50 cells a polar differentiation had not yet taken place. It must 

 be assumed, therefore, that the greater flattening of the cells 

 at one pole as compared with those at the other is delayed in 

 some cases. In. general however, the polar differentiation may 

 be saidfirst to manifest itself at about the 50-cell stage or almost 

 immediately upon the completion of the blastocyst wall. 



Occasionally one or more large cells occur at one pole of the 

 egg (text fig. 5 K) . Figure 25 is a blastocysts consisting of 52 cells. 

 The large cell in the section is one of four similar cells in juxta- 

 position at this pole. Perhaps these represent a somewhat re- 

 tarded generation of blastomeres. The case brings to mind such 

 a cell in an advanced blastocyst already in process of forming 

 the entoderm (figs. 37 and 38). Occasionally these larger cells 

 situated in the wall are darker and have their long axes radially 

 arranged (text fig. 5 K). Sometimes they are found in process 

 of division. 



In the opossum egg there is no polarity due to the presence 

 of a yolk mass at the 'formative' pole, as in Dasyurus, nor to 

 the presence of included cells at the 'entodermal' poles, as Selenka 

 would have it. In a few cases, however, where there is a great 

 attenuation of the cells at one pole of these early blastocysts, 

 such thinning away may be due to the presence of large numbers 

 of included cells. The matter of included cells will be discussed 

 below. 



3) Inclusion within the blastocyst cavity. There may always be 

 distinguished within the blastocyst cavity such inclusions as 

 were mentioned in connection with the 16-celled stage (figs. 25 

 and 26). Masses of coagulum (precipitated proteins of the 

 blastocystic fluid) are seen in one part or the other of the cavity. 

 Yolk granules are scattered in varying quantity throughout the 

 cavity and are nearly always surrounded with cytoplasmic frag- 

 ments. Most of these are the result of yolk elimination from 



